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9 elimination rxns

1.
Elimination Reactions

2.
• Elimination reactions involve the loss of elements fromthe starting material to form a new π bond in theproduct.General Features of Elimination

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• Equations [1] and [2] illustrate examples of eliminationreactions. In both reactions a base removes theelements of an acid, HX, from the organic startingmaterial.Elimination Reaction with Strong Bases

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• Removal of the elements HX is calleddehydrohalogenation.• Dehydrohalogenation is an example of β elimination.• The curved arrow formalism shown below illustrateshow four bonds are broken or formed in the process.Dehydrohalogenation

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• The most common bases used in elimination reactionsare negatively charged oxygen compounds, such as HO¯and its alkyl derivatives, RO¯, called alkoxides.Bases used to Promote Elimination Rxns.

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• To draw any product of dehydrohalogenation—Find theα carbon. Identify all β carbons with H atoms. Removethe elements of H and X form the α and β carbons andform a π bond.Products of Elimination

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• Recall that the double bond of an alkene consists of a σbond and a π bond.Alkenes—The Products of Elimination

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• Alkenes are classified according to the number ofcarbon atoms bonded to the carbons of the double bond.Classifying Alkenes

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• Recall that rotation about double bonds is restricted.No Free Rotation Around a Double Bond!

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• Because of restricted rotation, two stereoisomers of 2-butene are possible. cis-2-Butene and trans-2-butene arediastereomers, because they are stereoisomers that arenot mirror images of each other.Diastereomers

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• In general, trans alkenes are more stable than cisalkenes because the groups bonded to the double bondcarbons are further apart, reducing steric interactions.Cis/Trans-Alkenes (E/Z)(E) Alkene(Z) Alkene

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• The stability of an alkene increases as the number of Rgroups bonded to the double bond carbons increases.Order of Alkene Stability• The higher the percent s-character, the more readily an atomaccepts electron density. Thus, sp2carbons are more able toaccept electron density and sp3carbons are more able todonate electron density.• Consequently, increasing the number of electron donatinggroups on a carbon atom able to accept electron densitymakes the alkene more stable.

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• There are two mechanisms of elimination—E2 and E1.• E2 mechanism—bimolecular elimination• E1 mechanism—unimolecular elimination• The E2 and E1 mechanisms differ in the timing of bondcleavage and bond formation, analogous to the SN2 andSN1 mechanisms.• E2 and SN2 reactions have some features in common, asdo E1 and SN1 reactions.Mechanisms of Elimination

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• The most common mechanism for dehydrohalogenationis the E2 mechanism.• It exhibits second-order kinetics, and both the alkylhalide and the base appear in the rate equation i.e.E2 Mechanismrate = k[(CH3)3CBr][¯OH]• The reaction is concerted - all bonds are broken andformed in a single step.

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• There are close parallels between E2 and SN2 mechanisms inhow the identity of the base, the leaving group and thesolvent affect the rate.• The base appears in the rate equation, so the rate of the E2reaction increases as the strength of the base increases.• E2 reactions are generally run with strong, negatively chargedbases like¯OH and ¯OR. Two strong sterically hinderednitrogen bases called DBN and DBU are also sometimes used.Strong, Bulky Bases Favor E2 Mechanism

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• The SN2 and E2 mechanisms differ in how the R groupaffects the reaction rate.Differences Between E2 and SN2 Reactions

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• The increase in E2 reaction rate with increasing alkylsubstitution can be rationalized in terms of transition statestability.• In the transition state, the double bond is partially formed.Thus, increasing the stability of the double bond with alkylsubstituents stabilizes the transition state (i.e. lowers Ea,which increases the rate of the reaction according to theHammond postulate).Increasing Substitution Increases rate of E2 Reaction

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• Increasing the number of R groups on the carbon with theleaving group forms more highly substituted, more stablealkenes in E2 reactions.• In the reactions below, since the disubstituted alkene is morestable, the 30alkyl halide reacts faster than the 10alkyl halide.Comparing E2 Reactions

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• Recall that when alkyl halides have two or more different βcarbons, more than one alkene product is formed.• The major product is the more stable product—the one withthe more substituted double bond.• This phenomenon is called the Zaitsev rule.The Zaitsev (Saytzeff) Rule

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• A reaction is regioselective when it yields predominantly orexclusively one constitutional isomer when more than one ispossible. Thus, the E2 reaction is regioselective.The Zaitsev (Saytzeff) Rule

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• When a mixture of stereoisomers is possible from adehydrohalogenation, the major product is the more stablestereoisomer.• A reaction is stereoselective when it forms predominantly orexclusively one stereoisomer when two or more are possible.• The E2 reaction is stereoselective because one stereoisomeris formed preferentially.Stereoselective E2 Reactions

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• The dehydrohalogenation of (CH3)3CI with H2O to form(CH3)C=CH2 can be used to illustrate the second generalmechanism of elimination, the E1 mechanism.• An E1 reaction exhibits first-order kinetics:The E1 Mechanismsrate = k[(CH3)3CI]• The E1 reaction proceed via a two-step mechanism: the bondto the leaving group breaks first before the π bond is formed.The slow step is unimolecular, involving only the alkyl halide.• The E1 and E2 mechanisms both involve the same number ofbonds broken and formed. The only difference is timing. In anE1, the leaving group comes off before the β proton isremoved, and the reaction occurs in two steps. In an E2reaction, the leaving group comes off as the β proton isremoved, and the reaction occurs in one step.

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• The rate of an E1 reaction increases as the number of R groupson the carbon with the leaving group increases.Increasing the Rate of an E1 Reaction• The strength of the base usually determines whether a reactionfollows the E1 or E2 mechanism. Strong bases like ¯OH and ¯ORfavor E2 reactions, whereas weaker bases like H2O and ROHfavor E1 reactions.

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• SN1 and E1 reactions have exactly the same first step—formationof a carbocation. They differ in what happens to the carbocation.SN1 vs E1 Reactions• Because E1 reactions often occur with a competing SN1 reaction,E1 reactions of alkyl halides are much less useful than E2reactions.

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• The transition state of an E2 reaction consists of four atoms froman alkyl halide—one hydrogen atom, two carbon atoms, and theleaving group (X)—all aligned in a plane. There are two ways forthe C—H and C—X bonds to be coplanar.Stereochemistry of the E2 Reaction

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Anti Conformation Preferred for the E2 Reaction• E2 elimination occurs most often in the anti periplanar geometry.This arrangement allows the molecule to react in the lowerenergy staggered conformation, and allows the incoming baseand leaving group to be further away from each other.

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• The stereochemical requirement of an anti periplanar geometryin an E2 reaction has important consequences for compoundscontaining six-membered rings.• Consider chlorocyclohexane which exists as two chairconformers. Conformer A is preferred since the bulkier Cl groupis in the equatorial position.Elimination in Cyclic Systems• For E2 elimination, the C-Cl bond must be anti periplanar to theC—H bond on a β carbon, and this occurs only when the H andCl atoms are both in the axial position. The requirement for transdiaxial geometry means that elimination must occur from theless stable conformer, B.

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For Elimination, the Cl group must be in the in theaxial position of Cyclohexane Ring

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Consider the E2 dehydrohalogenation of cis- andtrans-1-chloro-2-methylcyclohexane• This cis isomer exists as two conformers, A and B, each ofwhich as one group axial and one group equatorial. E2 reactionmust occur from conformer B, which contains an axial Cl atom.

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• Because conformer B has two different axial βhydrogens, labeled Ha and Hb, E2 reaction occurs in twodifferent directions to afford two alkenes.• The major product contains the more stabletrisubstituted double bond, as predicted by the Zaitsevrule.E2 Reaction with the Cis Isomer

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• The trans isomer of 1-chloro-2-methylcyclohexane existsas two conformers: C, having two equatorial substituents,and D, having two axial substituents.E2 Reaction with the Trans Isomer• E2 reaction must occur from D, since it contains an axial Clatom.

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• Because conformer D has only one axial β H, E2 reactionoccurs only in one direction to afford a single product. Thisis not predicted by the Zaitzev rule.Result of the E2 Elimination

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• Two elimination reactions are needed to remove twomoles of HX from a dihalide substrate.• Two different starting materials can be used—a vicinaldihalide or a geminal dihalide.E2 Reactions and Alkyne Synthesis

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• Stronger bases are needed to synthesize alkynes bydehydrohalogenation than are needed to synthesizealkenes.• The typical base used is ¯NH2 (amide), used as thesodium salt of NaNH2. KOC(CH3)3 can also be used withDMSO as solvent.Types of Bases needed for Alkyne Synthesis

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• The reason that stronger bases are needed for thisdehydrohalogenation is that the transition state for thesecond elimination reaction includes partial cleavage ofthe C—H bond. In this case however, the carbon atom issp2hybridized and sp2hybridized C—H bonds arestronger than sp3hybridized C—H bonds. As a result, astronger base is needed to cleave this bond.E2 Reactions of Alkenes

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• The strength of the base is the most important factor indetermining the mechanism for elimination. Strongbases favor the E2 mechanism. Weak bases favor the E1mechanism.When is the Mechanism E1 or E2